The leader of the UK's MI5 domestic security service on Tuesday said that Iran is expanding the scope of the hybrid attacks that it coordinates in the UK and elsewhere.

In an annual address outlining his assessment of threats to the UK, MI5 chief Ken McCallum said a "major shift this year has been Iranian ambitions to come at harder targets, like military sites."

The remarks come after the US removed about a dozen long-range bombers from Fairford air base in the southern UK last week following an incident on September 27 during which several men were arrested on "suspicion of terrorism offenses." An investigation is ongoing.

Uptick in 'Iran-linked operations'

McCallum said these kinds of incidents were "not unique," warning that Tehran is seeking targets beyond Iranian diaspora living abroad who oppose the regime.

These include Iranian media organizations, Israeli interests in the UK, and arson attacks on Jewish sites that British authorities say were carried out by Iranian proxies.

The spy chief warned of an "unflinching response" to "those who seek to cause mayhem on British streets – whether hired thugs here or their paymasters overseas."

He added that more than 50 people have been arrested in "Iran-linked operations" in the UK over the past year.

However, McCallum said Iran's ability to carry out more severe and complex attacks using explosive drones remains limited by the sophistication of the proxies and criminals it has used ⁠for operations in the UK.

UK says Fairford air base incident likely coordinated by Iran 

On Monday, UK Prime Minister Andy Burnham said the suspected plot at Fairford air base was coordinated in Iran and was "almost certainly" directed by Iran's Islamic Revolutionary Guard (IRGC).

The degree of competence exhibited and the threat posed by the group of young British nationals from London is not entirely clear but seems potentially limited. Seven suspects are currently released on bail under strict conditions pending the investigation

Preliminary reports suggest that despite a widespread security alert and bomb squad deployment the day of the incident, investigators ultimately did not find viable explosives in the "suspicious vehicles" that were reported near the base. They did, however, find accelerants like petroleum.  

"We will crack down on Iran's use of criminal networks, using our full range of powers to target them and their funding streams, and we will bring further sanctions to restrict the capacity of Iran and the IRGC to do us harm," Burnham said. 

Foreign Secretary Ed Miliband said on Monday that his department had summoned Iran's ambassador to "reiterate" the message that Britain "will not tolerate attacks on our soil." 

He named Ruhollah Bazghandi, head of counterespionage operations in the IRGC and Shahram Ali Reza Tamarzadeh Jakki, an alleged facilitator for the Zindashti criminal network. 

Miliband said they were being sanctioned for "their role in facilitating or engaging in hostile activity on behalf of the Iranian regime, with the clear intent of threatening, planning or conducting attacks overseas."

Tehran has accusations of involvement in the Fairford incident, along with other accusations of coordinating attacks in the UK.

RAF Fairford is a major military air field fairly close to the south coast of England. It's a rare European base from which the US operates heavy bombers, with the range to reach Iran amid the current conflict in the Gulf.

Edited by: Wesley Dockery